I don't run Zen Cart locally, so I don't get to see the changes unless I upload them to the server and take a look.

To edit php files I generally use Notepad (although Notepad++ is recommended) or Crimson Editor.

If I'm trying to change the stylesheet (css) I use Firefox with the webdeveloper addon to tinker with the stylesheet and see the changes immediately. I then copy and paste the changed stylesheet into one of the text editors I mentioned above, save it locally and FTP it to the site.

Just a note on my earlier post (from 2008), SmartFTP isn't free anymore, so go with Filezilla.